Casing Integrity in Hydrate Bearing Sediments

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Casing Integrity in Hydrate Bearing Sediments
Wealth from Oceans

• Reem Freij-Ayoub, Principal Research Engineer
• CESRE

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Well integrity in hydrate bearing sediments (HBS)
JIP sponsors
Shell Global Solutions
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Outline

• What are gas hydrates
• Possible drilling and well completion problems
• The model
• The dissociation algorithm
• Cases studied
• Results
• Conclusions/Future work

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Gas hydrates

• Ice-like structures composed of water and natural
  gas molecules
• Under conditions of high pressure and low
  temperature, water molecules form cages which
  encapsulate gas molecules inside a
  hydrogen-bonded solid lattice
• Large gas storage capacity1 volume of gas
  hydrate contains up to 180 volumes of gas at stp

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Drilling and well completion problems
Hole enlargement
Casing collapse
Gas hydrate-related drilling problems (Adapted
from Maurer Engineering, Inc.)
Gas hydrate-related casing problems (Adapted from
Maurer Engineering, Inc.)
Hydrate dissociation? Gas release (gasified
mud) ? Loss of cohesion
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Model

• Casing heating
• During drilling of lower sections of the wellbore
  or
• During production

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Dissociation algorithm
Porosity
F current porosity Fo initial porosity Vc
volume of the Structure I crystal
Pore Pressure
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Strength degradation with hydrate dissociation
Friction angle
porosity in percent ().
UCS, cohesion, tensile strength
C is cohesion in MPa UCS is unconfined
compressive strength in MPa F is angle of
internal friction in degrees (o) is
tensile strength in MPa
Tan et al. (2005)
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Well integrity in HBS
Geomechanical strength-petrophysical correlations
Hydrate dissociation strength degradation
algorithm
In situ stress PP formation, cement casing
strength
Heat transfer into the formation
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Model and cases studied
Cases studied-Symbol Cement Strength Hydrates
Case 1 strong cement and no hydrates in sediments (S-NH). strong no
Case 2 weak cement and no hydrates in sediments (W-NH). weak no
Case 3 strong cement with hydrate bearing sediments (S-H). strong yes
Case 4 weak cement with hydrate bearing sediments (W-H). weak yes
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Formation properties
Parameter Value
Biots coefficient 1.0
Sediment porosity 0.25
Sediment porosity of the middle layer after hydrate dissociation 0.4
Thermal conductivity 1.4 Wm-1K-1
Specific heat capacity 1.9 x103 JK-1kg-1
Linear thermal expansion coefficient of hydrates 7.7x10-5 K-1
Linear thermal expansion coefficient of pore fluid 30x10-5?K-1
sediment solid dry density 2.800 kgm-3
Water density 1030 kgm-3
Modulus of Elasticity of sediments 807.6 MPa
Poissons ratio 0.35
Cohesion 1.7 MPA
Angle of internal friction 33.17º
Tensile strength 0.53 MPa
Water depth 800 m
Top level of hydrate layer below seabed 20 m
Bottom level of hydrate layer below seabed 60 m
In-situ temperature 15 ºC (288 K)
Gas constant 8.31441 JK-1Mol-1
Hydrates crystal volume 1.728x10-27 m3
Avogadro number 6.02205x1023
In situ stress ratio 1
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Cement and casing properties
Parameter Value
Casing properties
Linear thermal expansion coefficient of steel 37x10-7?K-1
Thermal conductivity 1.4 Wm-1K-1
Casing thickness 0.635in
Casing Poissons ratio 0.3
Youngs modulus of steel 210 GPa
Casing yield stress 379 MPa
Casing external diameter 20 in
Cement-casing bond properties
Coupling spring tensile strength limit 1x1020 MPa
Coupling spring cohesion limit 1x1020 MPa
Coupling spring friction angle 0º
Initial temperature 3 ºC (288 K)
Casing raised temperature 33 ºC (298 K)
Casing density 7.85 x103 kg m-3
Casing top axial load 1.37 million Pound Force
Cement thermal properties
Thermal conductivity 0.66 Wm-1K-1
Specific heat 1.9x103 JK-1kg-1
Thermal expansion 7.7x10-5 K-1
Strong cement
Modulus of elasticity 55.16 GPa
Poissons ratio 0.4
cohesion 11.4 MPa
Friction angle 10º
Tensile strength 2.6 MPa
Weak cement
Modulus of elasticity 807.6 MPa
Poissons ratio 0.35
Cohesion 1.7 MPa.
Friction angle 10º
Tensile strength 0.53 MPa
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Von Mises stress
at t0
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Maximum von Mises stress
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Ellipse of plasticity
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Thrust
The profile of the thrust (normal force per
linear meter of casing length) along the casing
for the case of strong cement, and hydrates in
the sediments, tension positive compression
negative. The combined effect of heating the
casing and the dissociation of hydrates creates
compressive normal forces in the casing at the
interval where hydrates exist.
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Maximum thrust
The absolute maximum thrust (hoop stress) in the
casing decreases with heating. This maximum
thrust occurs at the top of the casing for all
the cases studied and is detected close to the
base of the hydrate layer in case of its presence
after 4 days of heating. No risk of hydrostatic
buckling is found.
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Seabed subsidence or heave
Before heating
After heating
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Formation yield
Maximum yield radius
Before heating
Location of maximum yield radius
After heating
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Conclusion

• In the scenario considered the casing remains
  safe.
• The main impact of heating the casing and
  dissociating the hydrates was on the formation
  integrity.
• It is necessary to consider fluid flow (one or
  two phase flow) and its impact on reducing the
  pressure on the casing. This requires certain
  assumptions about the permeability of the cement
  and whether it will serve as a flow channel or
  not. Such a model should also allow for the
  reformation of hydrates.
• The accurate consideration of fluid flow requires
  modelling crack growth which can be done in a
  discrete modelling code.
• It is important to examine the effects of the
  depth-proximity to seabed- and thickness of the
  hydrate layer.

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Future work

• Model Fault reactivation and flow through faults
  using ABAQUS

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